Processing of sewage sludge into humus

ABSTRACT

The invention describes a process for the conversion of sewage sludge to humus by a complete filling cycle in a way that in a first cycle a primary polder is filled up to a stated height with sewage sludge. After that, the water content of the liquid sewage sludge in the primary polder is reduced, preferably by drainage, and a seeding with fast-growing plants with fine roots, preferably grass, is carried out. The plants in the primary polder are then subjected to a period of growth that concludes the first cycle. It is possible to use several polders parallel at different stages of the cycle. One cycle takes approximately 12 months.

BACKGROUND ART

The invention concerns a process for the conversion of sewage sludge tohumus as well as a polder for the carrying out of the process.

Large parts of the population have reservations concerning the use ofsewage sludge as a fertilizer and for the improvement of the ground.Although legislation attempts to ensure the general public that thereare ordinances for indirect polluters of public sewage sludge, parts ofthe public fear an accumulation of harmful substances in the ground.

Furthermore, the surplus sludge of the waste water treatment plant is ofvery liquid consistency (99% water). In order to keep the transportationcosts within limits it is necessary to carry out expensive dehydrationprocesses.

One known process for the conversion of sludge to humus applies reeds(Phragmatis). These plants, however, are very expensive as they must becultivated in bales. Furthermore, the strong stalks and roots of thereed require increased working before the converted sludge can beprocessed. Due to the regular filling with sludge in the known processthe degree of complete drying of the converted sludge is relatively low.As the reed plants are used over the entire filling period of 10-15years the percentage of decomposed plants is relatively low.

German Patent No. DE 43 23 610 describes a bio-polder (interim storage)for the biological clarification and/or mineralization of soil andsludge and for the biological clarification of waste water that ispolluted by organic and inorganic substances. The bio-polder usuallycontains at least one aerobic and one anaerobic layer that lie on top ofeach other. The harmful substances that have not been eliminated in theaerobic layer are drawn into the anaerobic layer by gravity and aresubjected to successive oxidative and reducing processes.

The invention is the result of the task to find a process for theconversion of sewage sludge to humus avoiding chemicals. This task ismet for the process for the conversion of sludge to humus by thecharacteristics as stated in the describing part of claim 1.

Further development and advantageous arrangement of the invention arisefrom the further claims, the description, and the drawings.

SUMMARY OF INVENTION

The invented process for the conversion of sludge to humus ischaracterized by the following steps:

a. Filling with liquid sewage sludge in a primary polder up to a primaryfilling height;

b. Drying of the liquid sewage sludge preferably by drainage evaporationand/or discharge of the impounding water;

c. Seeding of fast-growing plants with fine roots, preferably grass;

d. Initiation of a growing phase of the plants for a stated period oftime;

e. Repetition of the preceding steps until the polder is filledcompletely; and

f. Final resting of the sludge and emptying the polder.

The invented process is a low-cost process for the dehydration of sludgethat in comparison to mechanical processes reduces the quantity of thesludge. The invented use of fast-growing plants with fine roots resultsin a fine and even root penetration, an accumulation of micro-organisms,and a reduction of harmful substances (a PCB--polycyclic biphenol, AOX,PAK).

In addition, the sludge that is covered with grass decompose in ashorter period of time than can be achieved by the known process withreed. Furthermore, the use of fast-growing plants with fine roots hasthe advantage that the low height of the plants favors ultravioletradiation. This results in a remarkable reduction of salmonella orphatalates (softening agents) that are contained in the sludge. Thedegree of hyginisation is considerable.

The final product is dry, crumbly, dark-brown soil that is completelyfree of waste water odors and is characterized by good physical featuresfor the improvement of soil. Furthermore, a remarkable reduction of thevolume can be achieved.

Due to the use of fast-growing plants with fine roots cycles of theconversion can be achieved that are clearly shorter than by the knownprocess with reed. In addition, the invented process is cost-savingcompared to the known process.

Furthermore, the low depth of the filling layers and the long restingperiods of the sludge result in a better aeration of the soil in theinvented process.

In a further arrangement of the invented process, the single steps ofthe process are carried out in parallel in further polders. Thus,interruption of the filling process can be avoided. It is preferred touse three polders: The filling of liquid sewage sludge in polder I canbe carried out at the same time as the drying of the sludge and theseeding with fast-growing plants with fine roots in polder II and thegrowing of the plants in polder III.

The plants can be mowed for an additional reduction of harmfulsubstances. Harmful substances from the sewage sludge accumulate in themowed grass and can be concentrated, for instance, by combustion. Thisresults in a considerable reduction of e.g. heavy metals in the finalproduct that has been converted to humus.

A polder for the invented process that is suitable for the carrying outof the invented process consists of waterproof side walls and a bottomplate. A pipe or surface drainage is installed above the bottom plate.

A further arrangement of the invented process provides for a polder thatis suited for the invented process that contains a water blocking orhardly permeable layer towards the subsoil and the slopes. This layercan be made of clay or PE-foil. A pipe or surface drainage is installedabove this layer and can be protected by a layer of pebbles with a coverof a mixture of minerals or permeable concrete stones. A sewer buildingwith a gate valve that is connected to the polder should be provided forthe discharge of impounding water. A waste water treatment plant issupplied by the drainage water and/or the impounding water overpipelines.

A further process engineering arrangement takes into consideration thatunder certain climatic conditions activated soil can lose the content ofmicro-organisms and its crumbly structure if kept airtight. In theinvented process as described above a new layer of sludge and water isapplied after the conversion to humus, so that the first layer is keptairtight until the new layer starts to dry and form cracks. This appliesespecially during longer periods of humid weather.

In order to maintain the good physical and biological consistency of thefirst layer, i.e. the layer flooded by a layer of sludge and water, thefirst layer, i.e. the flooded layer, can be moved to the slope at theside of the polder by appropriate tools, e.g. a caterpillar, after theconversion process of this layer. Thus, only a small portion of theconverted substrate is flooded with water during the next fillingperiod.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in the following from the drawings.

FIG. 1 is a first arrangement of an invented polder;

FIG. 2 is a second arrangement of an invented polder;

FIG. 3 illustrates a filling cycle;

FIG. 4 illustrates an arrangement of polders;

FIG. 5 illustrates a sewer building; and

FIG. 6 illustrates a top view of the sewer building.

DISCLOSURE OF INVENTION

FIG. 1 shows a first arrangement of an invented polder 1. A polder 1 isconstructed, for instance, with concrete walls. The bottom plate isequipped with a pipe or surface drainage 3 and covered by a layer ofpebbles 2. This layer 2 is preferably covered by a mixture of mineralsor concrete stones 4. A sewer building 6 with a gate valve 12 can beconnected to the walls of the polder 1. In addition to the pipe orsurface drainage 3, this sewer building 6 can supply a waste watertreatment plant with impounding water. The gate valve 12 is lowered intoa trap 7 for the regulation of the impounding water. The sewer building6 can be connected to the drainage 3. A breakthrough 5 through the wallsof the polder 1 leads the impounding water to the sewer building 6. Theheight of the breakthrough 5 can be changed by the gate valve 12.

A possible arrangement of a sewer building 6 as described in FIG. 1 isdepicted in FIG. 5 and FIG. 6. The sewer building 36 of FIG. 5 is restedon pedestals 40, 41. It is equipped with an outlet pipe 37 that ispreferably connected to the successing waste water treatment plant theside of the sewer building that is opposite the outlet pipe 37 isconnected to the interior of the polder. The polder is connectedespecially with a drainage pipe 32 to the sewer building 36. A layer ofpebbles 31 and a (33) cover are installed above the drainage pipe 32.The sewer building 36 is equipped with an opening 34 towards the polder.The height of the opening 34 can be changed by a valve 35. The openingof the valve 35 can be changed by operating a valve 38, 39.

FIG. 6 shows a sewer building 43 in top view. The size of an opening 44can be changed by a valve 42. The sewer building 43 is preferablyconstructed rectangularly.

FIG. 2 shows a second arrangement of an invented polder 16. The polder16 is equipped with a water blocking or hardly permeable layer towardsthe subsoil 17 and slopes 9. A pipe or surface drainage 11 is installedabove this layer. The pipe or surface drainage 11 is protected by alayer of pebbles 10 with a cover 14 of a mixture of minerals or concretestones. A sewer building 13 with gate valve 15 that is connected to thepolder should be provided for the discharge of impounding water. A wastewater treatment plant is supplied by the drainage water and/or theimpounding water over pipelines 8.

FIG. 3 shows the complete filling cycle 25 to 30. During the first cycle25, polder I is filled with sewage sludge up to height A. In the sameperiod that polder II is filled with sewage sludge, a reduction of thewater content of the liquid sludge--preferably by drainage, evaporation,and/or discharge of the impounding water--and the seeding withfast-growing plants with fine roots - preferably grass--together withthe initiation of the growing period is carried out in polder I. Afterpolder II is filled with sewage sludge, polder m is filled with sewagesludge up to height A. The plants in polder I are subjected to a periodof growth whereas a reduction of the water content as well as theseeding and the initiation of the growing period takes place in polderI. That concludes the first cycle. Each polder I-III requiresapproximately 12 months for one cycle. The cycle can be shortened orextended according to the circumstances, e.g. weather. Cycle 25 isfollowed by further cycles 26 to 29 up to the respective filling heightsB to E until the polders I--III have been filled completely. Then themaximum filling height of the polders I-III has been reached. The finalfilling cycle 29 is followed by another cycle 30. In that cycle 30 thesludge is either stored or the polders I-III are emptied and filledanew.

FIG. 4 shows the arrangement with 3 polders 20 to 22. The depths of thelayers 22 to 25 are stated. Due to the proceeding conversion to humusand the resulting reduction of quantity, they do not turn out to be thatdeep, however.

The invention is not restricted to the examples described. The inventedsolution covers the use of one polder only or of more than threepolders. The polders that have been described as examples have not yetbeen described finally, either. Their arrangement and design can bemodified by using other designs, for instance, of the drainage, thecover, or the sewer building, or by not using single elements.

What is claimed is:
 1. A process for the conversion of sewage sludge tohumus, including the following steps:a. filling with liquid sewagesludge in a primary polder up to a primary filling height; b.dehydration of the liquid sewage sludge preferably by drainage,evaporation, or discharge of the impounding water; c. seedlings offast-growing plants with fine roots; d. initiation of a growing phase ofthe plants for a stated period of time or up to a stated degree ofgrowth; e. repetition of the preceding steps until the polder is filledcompletely; and f. final resting of the polder and emptying of thepolder.
 2. Process as defined in claim 1, wherein the steps of theprocess are carried out in additional polders, but in a changed order ofrank.
 3. Process as defined in claim 1, wherein the plants are mowed fora reduction of harmful substances and the mowed grass is storedseparately or exterminated.
 4. Polder to carry out the process asdefined in claim 1, characterized by the fact that the sidewalls andbottom plates of polder are constructed waterproof, that a pipe orsurface drainage is installed above the bottom plate that can beprotected by a foyer of pebbles with a cover of a mixture of minerals orpermeable concrete stones, that a sewer building with a gate valve forthe discharge of the impounding water is connected with the polder andthat a waste water treatment plant is supplied with the drainage wateror impounding water over pipelines.
 5. Polder to carry out the processas defined in claim 1, characterized by the fact that the polder isequipped with a water blocking or hardly permeable layer towards thesubsoil and the slopes, that a pipe or surface drainage is installedabove the subsoil or toward the slopes that can be protected by a layerof pebbles with a cover of a mixture of minerals or permeable concretestones, that a sewer building with a gate valve for the discharge of theimpounding water is connected with the polder, and that a waste watertreatment plant is supplied with the drainage water or impounding waterover pipelines.